Large scale (Megawatt class) wind turbines are becoming increasingly used as a source of renewable energy for utilities throughout the world. One approach to achieving efficient conversion of the mechanical power from the blades of a wind turbine into electrical energy supplied to a utility grid is the use of a doubly fed induction generator (DFIG) combined with a power electronics converter. The operation of such systems has been described in a number of publications, of which the following are representative:
Pena et al., “Doubly Fed Induction Generator Using Back-to-Back PWM Converters and Its Application to Variable Speed Wind-Energy Generation,” IEEE Proc.-Electr. Power Appl. 143(3):231-241, May 1996.
Rostoen et al., “Doubly Fed Induction Generator in a Wind Turbine,” Norwegian University of Science and Technology, 2002 (www.elkraft.ntnu.noleno/Papers2002/Rostoen.pdf).
Poitiers et al., “Control of a Doubly-Fed Induction Generator for Wind Energy Conversion Systems,” International Journal of Renewable Energy Engineering Vol. 3, No. 2, August 2001.
U.S. Pat. No. 4,994,684, Lauw et al., “Doubly Fed Generator Variable Speed Generation Control System,” Feb. 19, 1991.
The primary components of a representative DFIG system are a stator connected to the utility grid, an associated rotor connected to the wind turbine, rotor electrical connections through slip rings, a rotor side converter, a line side converter, a DC link connecting the two converters, and a controller for the converters.
The doubly fed induction generator system is generally quite well suited to variable speed wind turbine operation, but grid voltage variations can present a problem. For example, transient conditions on the utility grid may occur for short periods of time, such as a few cycles, or for longer periods of time. A common example is a sag or surge in the grid voltage. Previous systems have contemplated reacting to these instabilities by activating a command to drop the DFIG system off the line, i.e., to disconnect the generator from the utility grid. Thereafter, when the quality of the utility voltage is reinstated, the generator is restarted and output power is conditioned as necessary prior to reconnection to the grid.